Crankshafts in car engines are mass-produced parts with lot sizes that stretch into the millions. Forged steel crankshafts are preferred for crankshafts exposed to high loads, e.g., in diesel engines. Significantly cheaper than these are cast iron crankshafts, which tend to be used in the area of normally loaded motors. Technical progress in engine development has led to higher gas forces and consequently greater loads being placed on crankshafts, and at the same time bearings have been downsized for reasons of energy consumption.
As a result, the load capacity of the bearings of steel or cast iron crankshafts is further reduced. High-load bearings are sometimes used in the field of mixed friction, and the way that the engines of modern vehicles usually cut out when there is no load—start-stop cycling—causes additional wear on the bearings. The calculated bearing gap when the engine is running is within a range below 1 μm. High-load bearings have very precise geometrical forms and require a very low surface roughness due their chemical affinity to the metal of the bearing shell. A hardened running surface is advantageous for the transport and handling of the crankshaft and in terms of the bearing's load capacity. To benefit from these advantages, only a very low hardening depth of around 1/10 mm is necessary.
Hardening is also used to protect the oil holes.
However, not only crankshafts have bearing surfaces on the main and pin bearings and on the pin. Camshafts also have comparable bearing surfaces, which roll on the heads of the cylinder valves. In contrast to the bearing surfaces of the crankshaft, the bearing surfaces of the camshaft have a profile composed of a large circle and a small circle. The goal of increasing the strength of the bearing surfaces applies equally to both types of shaft.
The hardening of bearing surfaces by means of induction hardening or nitration, for example, is well known to the relevant professionals. The same applies to the equipment for deep rolling bearing radii, so that there is no need to provide a description of this equipment in relation to the invention. Deep rolling, in comparison with hardening, is a cost-effective and very environmentally-friendly process. In the case of deep rolling, the running surface is deformed and deep rolled under high pressure by a deep rolling tool made from tool steel or carbide. Deep rolled surfaces are characterized by positive compressive residual stresses with a depth in the mm range. Thanks to the deep rolling process, strength is guaranteed even after the subsequent finishing work, and the bearing running surface is more resistant to surface damage during assembly transport or during motor operation. As a result, wear on the bearing is reduced during motor operation. The sliding bearing shells on the counter-running surfaces thus enjoy a longer service life.
Hence the task of this invention is to strengthen the running surfaces on main and pin bearings, and on the crankpin diameter and flange diameter of crankshafts made from steel or cast iron or other metallic compositions, using deep rolling rather than hardening, and thereby to increase the service life of combustion engines. The strength of the oil holes is also increased by means of deep rolling rather than hardening.
The applicant is aware of a related technologies from JP 2006 34 6801 A1 and WO 2006 135 014 A1.
The publication JP 2006 34 6801 A1 explains how to improve the structure of the surface of the bearing pin of a crankshaft which has no lateral recesses, and how to finishing roll the bearing pin in a particular way, even if it has an oil hole. A pair of finishing rollers is provided for this purpose, which are designed in an identical disk shape. On their circumference, which rolls on the bearing pin, the finishing rollers have one to four projections, between which, in an axial direction, there are intermediate spaces of an equal size. The intermediate spaces between adjoining projections are larger than the inner-diameter of an oil hole. In the case of surfaces which have been finishing rolled in this way, a subsequent removal of material via machining with a low cutting depth can of course be dispensed with, so that a shaft structure is rolled into the bearing pin. The oil holes are deliberately not machined.
A method and a device for finishing rolling crankshafts are known from the publication WO 2006 135 014 A1. A pair of disk-shaped finishing rollers is placed on either side of the bearing pin of a crankshaft, opposite each other and at the same height. The crankshaft is caused to rotate by a source of power. The pair of finishing rollers are moved away from each other, while the bearing pin of the crankshaft is held by the outer circumference surface of the finishing rollers, which cause the finishing rolling of the crankshaft. In this publication, nothing is said about whether a surface structure is inserted in the finishing rolled bearing pin at the same time as the finishing rolling.
The task of the invention is achieved by deep rolling at least one bearing surface with at least one cylindrical deep roller, which has a surface structure and extends across the width of the bearing surface, and finally by machining the deep rolled bearing surface with a low cutting depth for the purpose of removing material.
Deep rolling is preferably carried out with three cylindrical deep rollers simultaneously.
Primarily the cylindrical surfaces on the main pins and crankpins of crankshafts are deep rolled. However, the flange of a crankshaft can also be machined in this way, in order to increase its surface strength and thereby prevent the occurrence of scores, which form during operation and through which oil can seep.
Even a crankshaft pin with an oil hole can be deep rolled in the same manner.
Primarily the main pin, the crankpin and the pins of crankshafts are deep rolled with a cylindrical deep roller which has a point-shaped swelling in the area of the oil hole.
For deep rolling, several cylindrical deep rollers are used, which have swellings arranged in a row along the width of the cylindrical surface being deep rolled.
After completion of the deep rolling process using deep rollers with a structured surface, further material removal via machining with a low cutting depth is required to restore the geometrical precision of the deep rolled surfaces. The bearing surfaces which have been reworked in this manner are finally burnished, for example, by means of finishing rolling or surface treatment with a laser beam.
A deep rolling tool for performing the process includes at least one cylindrical deep roller and at least one support element, which is located opposite the deep roller in relation to the profile of the bearing surface being deep rolled. The deep roller also features swellings or indentations on its cylindrical surface, which may run in the circumferential direction, the axial direction or diagonally to the axial direction.
Deep rollers with indentations in a diamond pattern on their cylindrical surface have proven particularly suitable. When several deep rollers are employed, they are situated opposite each other in relation to the profile of the bearing surfaces being deep rolled.
When three deep rollers are used in a deep rolling tool, it has proven advantageous when the deep rollers are situated in a triangular arrangement in relation to the profile of the bearing surface being deep rolled.
It is advantageous when the surface structures of at least two deep rollers follow one another. In the case of three deep rollers, one deep roller can be designed as a support which has a smooth cylindrical surface. However, one of the deep rollers should have a point-shaped swelling on its otherwise smooth cylindrical surface, which is situated at exactly the point that strikes an oil hole on the bearing surface when the deep roller is rolling on that surface.